Gasoline selective ZSM-5 FCC additives: effects of crystal size, SiO2/Al2O3, steaming, and other treatments on ZSM-5 diffusivity and selectivity in cracking of hexene/octene feed

ZSM-5 catalysts with silica/alumina ratios of 55/1 and 450/1, and with vary ing crystal size, were studied to better understand the operation of "gasol ine selective" FCC additives. Most materials were steamed for 5 h at 1400 d egreesF, and some were subjected to other treatments such as mechanical mil ling, P addition, and HCl reflux. These materials were characterized for to tal Al content, tetrahedral Al, and sorption capacity and rate. The ratios of octene isomerization to octene cracking, and of octene cracking to hexen e cracking, for a mixed hexene/octene feed at 538 degreesC were used to ass ess the potential FCC gasoline selectivity of these catalysts. Steaming generally led to a decrease in effective diffusivity, as calculate d from uptake rates. HCl reflux of two steamed samples removed substantial quantities of non-framework alumina, resulting in increased diffusivities a nd higher selectivities. Density of acid sites did not appear to be importa nt in determining the gasoline selectivity of the catalysts examined here. The effects of most catalyst treatments on reaction selectivities could be related to diffusional blockage, especially by non-framework alumina. Howev er, not all results could be explained by a simple reaction/diffusion model . Factors other than diffusion, such as narrowing of available reaction spa ce in the ZSM-5 channels, may also play a role in determining selectivities . (C) 2001 Elsevier Science B.V. All rights reserved.